Clamshell type mold tunnel apparatus using independent mold sections for production of extruded thermoplastic tubing, and particularly, profile tubing, have found use in the pipe forming industry. Descriptions of clamshell tubing apparatus are found in U.S. Pat. No. 4,439,130 Dickhut et al and U.S. Pat. No. 4,911,633, Comfort. These patents are incorporated herein by reference.
Clamshell type extrusion apparatus may produce profile pipe with undesirable surface irregularities. Such irregularities may impact on the appearance and or performance of the finished mold tubing product. For example, interior irregularities adversely impact on performance, e.g., by increasing flow turbulence. Exterior irregularities are undesirable on grounds of aesthetics. It has been suggested that such irregularities become more pronounced in large diameter profile tubing and double wall tubing produced by clamshell molding extrusion apparatus. Also it is reported that clamshell molding extrusion devices employing a "quick return" feature to reduce the number of individual mold blocks required for molding, suffer from alignment problems leading to undesirable irregularities. (A "quick return" feature in thermoplastic tubing molding apparatus is described in U.S. Pat. No. 4,504,206 issued to Lupke on Jan. 12, 1983, incorporated herein by reference.)
Misalignment is a common concern arising from the use of the prior art clamshell devices, whether direct drive of independent mold blocks or inter-linked chain drive. Individual clamshell mold sections are used to establish a travelling mold tunnel to profile a thermoplastic parison and form corrugated or ribbed pipe (See FIG. 2). Not only are the opposed mold block sections prone to separate from one another opposite the hinge connection, but also as a thermoplastic extrudate cools, it hardens and its viscosity increases, all within a travelling mold tunnel. These physical changes impart progressively increasing drag on the mold blocks and unless the extrudate is very thin, the additional, increasing drag tilts (cants) the mold blocks relative to one another. (See FIG. 3) That is, the unhinged portion of the mold block section opposite of the driving linkage, has a tendency to become misaligned (not square) relative to the immediately adjacent mold block and the mold tunnel. As the length to width ratio of the mold tunnel increases, the tilting/canting of one mold block creates a cascade effect where one mold block pushes against the two contiguous (front and back) mold blocks which in turn, may exert additional de-stabilizing force on the originally misaligned mold block section (refer to FIG. 3). Thus, the canting can become progressively worse along the mold tunnel length resulting in defective profiling of the molded thermoplastic tube. The prior art devices, as represented by U.S. Pat. No. 4,439,130 did not recognize the drag problem or the cascade effect of canting. Instead, referring to FIG. 1, the prior art relied on a combination of a guide G directly engaging the support carriage 24 and rollers R and cam followers C to not only pivot the clamshell sections between the open and closed positions but also to positionally stabilize the clamshell sections.